biophysical properties of skm
TRANSCRIPT
Dr G.A.KurhadeB.Sc,MBBS,DGO,MD
Sr.Lecturer (Physiology)Room No. 107
Biophysical Properties of skeletal muscles
Learning objectives:
• Differentiate fast and slow muscle fibres.
• Define term motor unit, muscle twitch ,muscle
recruitment,, summation, tetanus & treppe &
explain their functional significance of these in
everyday life.
• Discuss the length-tension relationship in
muscles.
• EMG and its uses.
Fast twitch sk ms. fibres
(type II A &IIB)
Slow twitch sk muscle fibres
(Type I)
Lateral rectus ms (occular) Soleus muscle.
•Gastrocnemius ms contains a mixture of fast and slow twitch
fibers.
•Exhibits a weighted average intermediate rate of tension
development on muscle stimulation.
•The basic structure of the myosin isoforms in both types is
similar (i.e., two heavy chains with two pairs of light chains).
•They are produced due to different genes & have different
amino acid sequences.
Fast twitch fibres
(WHITE)
Slow fibres (RED)
•Glycolytic enzymes activity ↑.
•Oxidative enzymes activity ↓.
•Less extensive blood vessels &
capillary system.
•Metabolic demand met by oxidative
phosphorylation.
•Extensive blood vessels & capillary
system.
Electron micrographs - few
mitochondria.
More mitochondria.
A much more extensive SR. Less extensive SR.
fatigue rapidly- (Because of the
dependence on glycolytic
metabolism)
Fatigue slowly.
These are used only occasionally &
for brief periods of time. (LR)
Used for more sustained activities
(e.g., maintenance of posture).
Red slow postural fibres Type II & white fast LR :
• Some fast fibers have both high glycolytic &
high oxidative capacities. Such fibers found in
mammals - uncommon in humans. (fight/flight)
• The fibers deriving their energy primarily from
oxidative phosphorylation (i.e., the slow type I
fibers & the fast type IIA fibers) contain
numerous mitochondria & high levels of the
oxygen-binding protein myoglobin.
• Because myoglobin is red –k/as "red fibers.” as
compared to white fast fibers.
Motor unit in red slow postural fibres & white fast LR :Characteristics Type I ( slow) red Type II (fast) white
Properties of nerve
Cell diameter Small Large
Conduction velocity Fast Very fast
Excitability High Low
Properties of muscle cells
Number of fibers Few Many
Fiber diameter Moderate Large
Force of unit Low High
Metabolic profile Oxidative Glycolytic
Contraction velocity Moderate Fast
Fatigability Low High
• Histochemical methods can be used to
distinguish the fiber types based on the activity
of the myosin ATPase.
• Typically, most skeletal muscles are a
intermix of fast fibers (types IIA and IIB) &
slow fibers (type I).
Difference between red slow postural fibres &
white fast LR contd…
Following muscle proteins are also expressed in a fiber type in specific manner:
• SERCA (Sarcoplasmic Endoplasmic Reticulum Calcium ATPase).
• The three troponin subunits (Tt, Ti, Tc,).
• Tropomyosin.
• C-protein.
Difference between red slow postural fibres & white
fast LR contd…
Differential expression of SERCA isoforms contributes
to the differences in the speed of relaxation between
fast & slow twitch muscle.
• SERCA1 in fast twitch muscle & SERCA2 in slow
twitch & cardiac muscle.
• The activity of SERCA1 > SERCA2.
• Ca2+ reuptake into the SR occurs more quickly in fast
muscles.
• HENCE these fibers have a faster relaxation time.
White (↑GE ) fast LR fibres (SERCA I -↑Ca++uptake) &
Red (OP ) slow postural fibres (SERCA II -↓ Ca++uptake)…
• Differential expression of troponin & tropomyosin isoforms influences the Ca2+ dependency of contraction.
• Troponin-C of fast fibers has two low-affinity Ca2+
binding sites & they begin to develop tension at higher [Ca2+ ] than do slow fibers because:
• Troponin-C of slow fibers has single low-affinity Ca2+-binding site.
• Whereas Troponin-T & tropomyosin isoforms also differ.
• Thus, the regulation of the Ca2+ dependence of contraction is complex, involving contributions from multiple proteins on the thin filament.
White (↑GE ) fast LR fibres (SERCA I -↑Ca++uptake) &
Red (OP ) slow postural fibres (SERCA II -↓ Ca++uptake) …
• Fast twitch muscle fibers can be converted into slow twitch muscle fibers (& vice versa), depending on the stimulation pattern.
• Chronic electrical stimulation of a fast twitch muscle results in the expression of slow twitch myosin &
↓ expression of the fast twitch myosin & ↑ in oxidative capacity.
• The mechanism(s) underlying this change in gene expression is unknown, but may be secondary to an elevation in resting intracellular[ Ca2+ ].
Characteristics of muscle:
• Excitability - responds to stimuli (e.g., nervous impulses)
• Contractility - able to shorten in length
• Extensibility - stretches when pulled
• Elasticity - tends to return to original shape & length after contraction or extension
Muscle twitch
Mechanical response of a single muscle fibre to a
single action potential- contraction followed by
relaxation.
Three phases:
• Latent period
• Period of contraction
• Period of relaxation
Muscle contraction :
Latent period :
• Following the action potential, there is a latent period, before the tension in muscle fibre increases.
• Latent period is longer in an isotonic twitch than in an isometric twitch.
• Latent period is positively correlated while velocity of shortening, duration of twitch & distance shortened are negatively correlated with load.
Period of contraction
Time interval from beginning of tension
development & peak tension- contraction time.
Cross bridges are active from the onset to the
peak of tension development & the myogram
tracing rises to a peak.
If the tension becomes great enough to overcome
the resistance (weight being moved) - the
muscle shortens.
Duration of contraction is longer in an isometric
twitch than in an isotonic twitch.
Period of relaxation
• After the contraction period of relaxation follows & is the result of [ca2+ ] returning to normal levels.
• The muscle tension ↓es to zero & the tracing returns to the baseline.
• If the muscle has shortened during contraction, it now returns to its initial length.
Isotonic and isometric contraction:
• The traditional preparations shown in figure usually use frog gastrocnemius muscles so they do not need to be kept warm and the muscle is tested in two modes:
• Isometric where the length of the muscle is not allowed to alter, & isotonic where the load
is kept constant.
• Isometric loading can be achieved in human subjects using a dynamometer –grip strength dynamometers being the commonest device.
• Isotonic loading is achieved by lifting weights slowly.
Graded muscle responses :
• Muscle contraction can be graded in two
ways:
1. By changing the speed (frequency) of
stimulation.
2. By changing the strength of the stimulus.
Factors affecting muscle force development
(a partial list)
• Muscle fibre type – fast / slow.
• Number of activated motor neurons
(recruitment).
• Frequency of discharge.
• Muscle length.
• Velocity of shortening/ lengthening
• Muscle geometry (physiological cross-sectional
area (PCSA), angle of pennation)
Stimulation frequency affects muscle force:
Tetanus-at stimulation frequencies >30/s (fused
tetanus)
Tetanus - force summation:
• Frequency at critical level-successive contractions rapid & fuse together- muscle contraction completely smooth & continuous-steady pull.
• The strength of contraction – maximum.
• Any additional ↑ in frequency beyond that point has no further effect in ↑ing contractile force. (all or none)
• Because enough Ca++ are maintained in the muscle sarcoplasm, even between AP, so that full contractile state is sustained without allowing any relaxation between the action potentials.
Recruitment-the size principle:• The stimulus frequency is not the only control
over the tension generated.
• The size of the stimulus controls the number of nerve fibres that get activated & hence the number of motor units that contract.
This effect can be seen in vivo .
• Electromyography (EMG) records the electrical activity of a muscle.
• If a muscle is contracting very weakly only a single motor unit may be activated.
• As tension ↑es additional motor units get recruited.
Recruitment –size principle• The order in which motor units get recruited is
not random.
• Smaller motor units get recruited first followed by larger ones .
• This makes sense because small motor units are
used for fine control which is required at low forces.
• At higher forces small changes in force are not necessary. (pushing a car compared to a book )
• If big motor units fired off force you would not be able to apply very small forces at all.
Length tension relationship:• The molecular mechanisms of muscle
contraction underlie & responsible for the biophysical properties of muscle.
• During contraction muscle:
a) Generate force (often measured as tension or stress)
b) ↓ in length – contract – shorten
• When studying the biophysical properties of muscle, one of these parameters is usually held constant, while the other is measured following an experimental maneuver.
• When a muscle is stimulated it does not instantly produce force.
• There is a delay in tension production after electrical activity is detected.
• The force builds up to its maximum fairly slowly.
• Similarly there is a lag in tension reduction after electrical activity ceases.
Active force development in the sarcomere depends
on actin-myosin overlap: Length-tension relationship• (A): no overlap between actin &
myosin, zero developed tension
• Between (A) & (B): tension ↑es
linearly as overlap ↑es.
• Between (B) & (C): maximum
overlap & maximum tension
• Left of (C): interference between actin filaments ↓es ability of cross bridges to develop tension.
• Left of (D): myosin filaments
collide with Z-lines & fold &
force declines rapidly.
Conclusion: Starling’s law
• THUS contractile force ↑es as the muscle length
is ↑ ed up to a point (designated L0 to indicate
optimal length).
• As the muscle is stretched beyond L0 - contractile
force decreases.
• i.e. At the resting length ( 2 μm -sarcomere). if
stimulated - muscle contracts with its maximum
force of contraction.
• This length-tension curve is consistent with the
sliding filament theory.
Length tension graphs for sarcomere compared
to whole muscle bundle.
Contractile components, SEC & PEC
• A muscle consists of an active force generating component & SEC &PEC connective tissue
component.
• SEC &PEC do not actively generate force but is stretched beyond its resting length it acts just like a rubber band and produces a passive, elastic force.
• The effect of both of these force generating elements on the actual force output of a muscle is shwon in picture.
Forcibly stretching a muscle well beyond its resting
length will generate a force higher than that produced
by active contraction.
Conclusion: ( Relation of length of Ms fibre to
force of contraction)
• The ↑ in active tension that develops during contraction ↓es as the muscle is stretched beyond its normal length i.e. a sarcomere length of > 2.2 μm.
• What is pre loaded & AL condition?
• Why is preloaded muscle more efficient than after loaded
• STARLING’S LAW???
• Its application in daily life???
Relation of velocity of contraction to load :
• You can lift a pen quickly but you cant lift 100 kg as quickly-
• Skeletal muscle contracts extremely rapidly (Vo) when it contracts against no load to a state of full contraction in about 0.1 second for average muscle.
• V0 corresponds to the maximal cycling rate of the cross-bridges [i.e., it is proportional to the maximal rate of energy turnover (ATPase activity) by myosin].
• The V0 for fast twitch muscle is higher than that for slow twitch muscle
Velocity of contraction Vs load
• ↑ing the load ↓es the velocity of muscle shortening
until, at maximal load, the muscle cannot lift the
load & hence cannot shorten (zero velocity)
• Further ↑ in load results in stretching the muscle (negative
velocity).
• If a muscle is contracting rapidly it cannot
generate as much force as when it is stationary.
• An even greater force is required to stretch a
maximally active muscle.
Velocity of contraction Vs load
• The maximum weight you can lift off your
chest rapidly is quite low.
• The maximum weight you can lift slowly is
somewhat higher, and the maximum weight
you can maintain the height of is higher still.
• An even higher weight will force you to lower
it slowly.
Hill's equation :• This is a popular state equation applicable to
skeletal muscle that has been stimulated to show
tetanus. It relates tension to velocity. The
equation is
• ( v+ b )(p + a)= b ( p o +a )
• Where P - load or tension in the muscle
• v - velocity of contraction
• P0 - maximum load or tension generated in the
muscle
• a & b - constants
( v+ b )(p + a)= b ( p o +a )
• Hill's equation demonstrates that the
relationship between P & v is hyperbolic.
• Therefore, the higher the load applied to the
muscle, the lower the contraction velocity.
• Similarly, the higher the contraction velocity,
the lower the tension in the muscle
Conclusion:
• With the ↑ in load the net force available to
cause velocity of shortening is ↓ed
correspondingly.
• i.e. why you can not push a car all alone.
• Isometric contraction-A
muscle applying force
without shortening.
• Concentric contraction
A muscle applying force
& shortening.
• Eccentric contraction -
A muscle applying force
but being extended
anyway.
• We do not think much about isometric activity but we use it all the time to maintain posture.
• Eccentric muscle activity is also common and is often used at the ends of activities to slow down movements.
• and is obviously used in situations when energy is being lost such as walking down stairs or landing from a jump.
Concentric activity what we normally think
about muscles doing:
A muscle applying force &
shortening
• Concentric (energy
generating, positive
work) contractions tend
to increase joint angular
velocity.
• Increase the total energy
of the system.
Concentric contractions on left side:
Eccentric muscle activity- A muscle applying
force but being extended anyway.
• Eccentric (energy absorbing, negative work) contractions tend to decrease (or brake) joint angular velocity.
• Reduce the total energy of the system.
Eccentric contractions-Eccentric -braking
contraction of m2 (on right side)
Isometric contraction
A muscle applying force without shortening-
Posture maintaining
Duration of isometric contraction- adapted to the
function the muscle performs:
activity.
LR, Lateral rectus muscle of the eye; G, gastrocnemius of the leg;
S, soleus muscle of the leg.
Beneficial effect –Chemical & mechanical
• Increasing the duration of the intracellular Ca2+
transient, as occur with tetany, provides the muscle with sufficient time to completely stretch the series elastic component with full contractile force of the actin-myosin interactions (i.e., maximal tension).
• Partial stretching of the SEC (as in a single twitch), followed by re stimulation of the muscle before complete relaxation yields an intermediate level of tension as in incomplete tetany.
• The location of SEC in skeletal muscle could be :
• Myosin molecule itself or the connective tissue of the endomysium, perimysium, & epimysium.
Maximum strength of contraction
• 50 lbs/sq inch= 3-4 Kg/sq cms of muscle.
• Quadriceps muscle can have 16 sq inches of
ms belly- so 800 pounds of tension can be
applied to patellar tendon.
• Hence muscles can pull their tendons out of
their insertions in bone.
OXYGEN DEBT• If the energy demands of exercise cannot be met by
oxidative phosphorylation, an O2 debt is incurred.
• After completion of exercise, respiration remains above resting level in order to "repay" this O2
debt.
• The extra O2 consumption during recovery phase is used to restore metabolite levels (such as creatine phosphate & ATP) & to metabolize the lactate generated by glycolysis.
• The ↑ cardiac & respiratory work during recovery ↑O2 consumption seen at this time & explains why more O2 has to be "repaid" than was "borrowed."
Rigor mortis:
• Several hours after death all the muscles of
body go into a state of contracture .
• i.e. muscles contract and become rigid even
without action potentials.
• CAUSE????